Exterior automotive body panels have been made in the past by spray painting sheet metal parts. Multi-layer paint coats, such as those referred to as clear coat/color coat paint systems, have been used to produce desirable optical effects. In addition to high gloss and high distinctness-of-image (DOI), these paint coats also are highly durable by providing chemical resistance, abrasion resistance and weatherability that avoids degradation by ultraviolet light.
In more recent years, molded plastic car body panels have been made with decorative clear coat/color coat paint films bonded to the molded plastic panel. Use of such films avoids certain environmental problems associated with evaporation of paint solvents while also reducing or eliminating the need for paint facilities and emission controls at the automotive production plant.
Because of the growing need to reduce atmospheric pollution caused by solvents emitted during the painting process, many different approaches have been taken in recent years for producing these decorative films. These processes can be generally categorized by solution-casting techniques or extrusion techniques. For instance, U.S. Pat. No. 5,707,697 to Spain et al. describes solution-casting techniques in which liquid-cast solvent-based clear coats and pigmented base coats are applied to a flexible casting sheet by a coating process such as reverse roll coating. The liquid-cast layers are separately applied and then dried at high temperatures to evaporate the solvents. Following the steps of casting the clear coat and color coat, the composite paint coat is removed from the carrier and transfer laminated to a thin, semi-rigid, thermoformable polymeric backing sheet. The preferred backing sheet is about 20 mils in thickness, although the backing sheet can have a thickness from about 10 mils to about 40 mils. The paint coated backing sheet is then thermoformed into a desired three-dimensional shape, followed by molding the substrate panel to the thermoformed sheet in an injection mold. This process, known as the "insert-mold" process, together with the transfer-lamination and thermoforming steps, are described in the above-mentioned '697 patent which is incorporated herein by this reference.
As an alternative to solvent-cast films, extruded films have been used for making exterior automotive clear coat/color coat paint films. International Application WO 96/40480 to Enlow et al., which is incorporated herein by this reference, describes an approach in which paint films and/or laminates are made by extrusion coating or coextrusion techniques. The paint film is transferred to a supportive backing sheet by transfer-lamination techniques, followed by thermoforming the paint coated backing sheet into a desired three-dimensional shape and injection cladding it to a substrate panel by the insert-mold process.
As a further alternative, a dry paint transfer sheet can be placed directly in an injection mold without thermoforming it outside the mold. The sheet in this instance is formed into a contoured shape under heat and pressure in the injection mold by the molding material in what is referred to as the "in-mold" process. Such a process, for example, is described in U.S. Pat. No. 4,810,540 to Ellison et al.
The present invention is based on a recognition that production costs for making these exterior automotive parts can be reduced by eliminating the injection-molding step. In other words, it can be desirable to manufacture an exterior automotive body member or panel so that the finished automotive part, such as a facia, is ready to put on the vehicle after the thermoforming step. This would require producing a polymeric substrate sheet with sufficient thickness and structural integrity to function as the finished part that adequately supports the transferred paint film. This also requires a polymeric substrate material that can be shaped by thermoforming techniques while not interfering with the desired optical properties (such as high distinctness-of-image and gloss) of the finished paint film. For instance, may be desirable to use polymeric substrate materials that contain high filler or regrind components in order to reduce the cost of the overall product. But these particulate materials can be transmitted from the substrate to the paint film during thermoforming, creating imperfections that degrade the otherwise desired smooth, high quality optical surface of the finished film. In addition, the thicker substrate sheet material can absorb substantial amounts of heat when bonding a paint film to it or when thermoforming it. Such excessive heat absorption can be transferred to the paint film surface and degrade the optical properties of the paint film by causing excessive haze or fogging of the clear coat. This problem is particularly critical when such exterior paint films are made from fluoropolymer resins, such as polyvinylidene fluoride (PVDF) and acrylic resin alloys of the types described in the above-mentioned '697 patent to Spain et al., International Application WO 96-40480 to Enlow et al., or the '540 patent to Ellison et al.